Re: NTNDArray array filter does not work on value

[Marty Kraimer via Tech-talk <tech-talk at aps.anl.gov>]

On 4/1/20 9:44 AM, Mark Rivers wrote:
> > mrk> pvget -r 'value[array=0:2]' PVRdoubleArray
> Can your proposed syntax work to limit all arrays to N values or do I need to specify it for each array?  If I use the proposed syntax can I get all elements of the NTNDArray while limiting all arrays to N values?
I was thinking of only providing support for individual fields.

The request syntax provides record options.
Maybe pvDatabaseCPP could support a record option?
-r 'record[array=low:high:inc]field(...)'

This would apply an array filter to all scalarArray fields in the 
structure the client is requesting.
The structure could have all field types, i.e. 
scalar,scalarArray,structure,structureArray,union,unionArray.
But ONLY scalarArray fields will have the filter applied.
I don't think it should be applied to structureArray or unionArray fields.
For complicated structures this can be an expensive request.

BUT this should only be used for get.
I see all kinds of bad behavior for puts.
> To my naïve thinking this is something that belongs as a simple option in the pvget and pvmonitor client and not something that requires a complex filter expression.  As a user I don't want to have to know the names of the NTNDArray structure elements in advance, I just want to know that the entire structure is called 13SIM1:Pva1:Image which I can determine from the OPI display for NDPluginPva.
The array filter can be used by any client, i.e. not just caget and caput.

I agree that it would be nice to have pvget provide a way to limit the 
amount of output for all scalarArrays in the received data.
Thus it could be used with all channel providers.
But note that is would be done on the client side so that entire arrays 
will still be passed from server to client.
Again I see all kinds of bad behavior if it is done for pvput.

Marty